Neural stimulation system providing auto adjustment of stimulus output as a function of sensed impedance

ABSTRACT

A neural stimulation system automatically corrects or adjusts the stimulus magnitude (stimulation energy) in order to maintain a comfortable and effective stimulation therapy. Because the changes in impedance associated with the electrode-tissue interface can indicate obstruction of current flow and positional lead displacement, lead impedance can indicate the quantity of electrical stimulation energy that should be delivered to the target neural tissue to provide corrective adjustment. Hence, a change in impedance or morphology of an impedance curve may be used in a feedback loop to indicate that the stimulation energy needs to be adjusted and the system can effectively auto correct the magnitude of stimulation energy to maintain a desired therapeutic effect.

This application is a continuation of U.S. application Ser. No.11/746,748, filed May 10, 2007 (now U.S. Pat. No. 7,742,823, issued onJun. 22, 2010), which is a continuation of U.S. application Ser. No.10/364,436, filed Feb. 11, 2003 (now U.S. Pat. No. 7,317,948, issued onJan. 8, 2008), which claims the benefit of U.S. Provisional ApplicationSer. No. 60/357,008, filed Feb. 12, 2002, the disclosure of which ishereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to neural stimulation systems and, moreparticularly, to an output control system used with such neural systemsthat automatically maintains the output of the stimulation system at acomfortable and efficacious level.

The present invention may be used in various stimulation therapies inwhich a neurostimulator is used to stimulate neural tissue. One examplewhere the present invention may be employed is with stimulation ofperipheral nerves, e.g., the nerves in the arms, legs, fingers, whichnerves are distant from the spinal cord. The present invention may alsobe used in stimulation of spinal cord nerves.

Spinal cord stimulation (SCS) systems, treat chronic pain by providingelectrical stimulation pulses through the electrodes of an electrodearray placed epidurally near a patient's spine. SCS is a well-acceptedclinical method for reducing pain in certain populations of patients.SCS systems typically include an Implantable Pulse Generator (IPG)coupled to an array of electrodes at or near the distal end of anelectrode lead. An electrode lead extension may also be used, if needed.The IPG generates electrical pulses that are delivered to neural tissue,e.g., the dorsal column fibers within the spinal cord, through theelectrodes of the electrode array. In an SCS system, for example, theelectrodes are implanted proximal to the dura mater of the spinal cord.Individual electrode contacts (the “electrodes”) may be arranged in adesired pattern and spacing in order to create an electrode array.Individual wires, or electrode leads, connect with each electrode in thearray. The electrode leads exit the spinal cord and attach to the IPG,either directly, or through one or more electrode lead extensions. Theelectrode lead extension, in turn, when used, is typically tunneledaround the torso of the patient to a subcutaneous pocket where the IPGis implanted.

The electrical pulses generated by the SCS stimulation system, or otherneural system, are also referred to as “stimulation pulses”. In an SCSsystem, the stimulation pulses typically have the effect of producing atingling sensation, also known as a paresthesia. The paresthesia helpsblock the chronic pain felt by the patient. The amplitude, or magnitude,of the stimulation pulses affects the intensity of the paresthesia feltby the patient. In general, it is desirable to have the amplitude ofstimulation comfortably set to a level which produces paresthesia toblock pain but not above a level that may actually result in pain apartfrom the native pain. Moreover, the stimulus amplitude should be set toa stimulus level lower than that which can recruit reflex motor nervesthat can cause involuntary muscle contractions.

SCS and other stimulation systems are known in the art. For example, animplantable electronic stimulator is disclosed in U.S. Pat. No.3,646,940 that provides timed sequenced electrical impulses to aplurality of electrodes. As another example, U.S. Pat. No. 3,724,467,teaches an electrode implant for neuro-stimulation of the spinal cord. Arelatively thin and flexible strip of biocompatible material is providedas a carrier on which a plurality of electrodes are formed. Theelectrodes are connected by a conductor, e.g., a lead body, to an RFreceiver, which is also implanted, and which is controlled by anexternal controller.

Representative techniques known in the art for providing for theautomatic adjustment of stimulation parameters of an implantablestimulator are disclosed, e.g., in U.S. Pat. Nos. 5,895,416; 5,735,887;5,333,618; and 4,735,204.

Patients having an SCS system have heretofore had to manually adjust theamplitude of the stimulation pulses produced by their SCS system inorder to maintain the paresthesia at a comfortable level. This isnecessary for a number of reasons. For example, postural changes, leadarray movement (acute and/or chronic), and scar tissue maturation, allaffect the intensity of the paresthesia felt by the patient. Because ofthese changes, i.e., because of postural changes, lead array movement,and scar tissue maturation, as well as other changes that may occur inthe patient, the paresthesia can be lost, or can be converted to painfulover-stimulation, thereby forcing the patient to manually adjust theoutput. There is a need for a method or system that would eliminate, orat least mitigate, the need to perform such manual adjustments. Suchmethod or system would be of great benefit to the patient.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing aneural stimulation system and method that automatically corrects oradjusts the stimulus amplitude in order to maintain a comfortable andeffective stimulation therapy. Auto correction of the stimulus amplitudeis linked to the measurement of lead impedance, i.e., the impedancemeasured between selected electrodes.

Because the events that lead to the necessity of an output amplitudechange are all associated with how much electrical energy is actuallydelivered to the neural tissue, and because how much energy is deliveredto the neural tissue is, in part, a function of the proximity of theelectrodes to the neural tissue, which in turn is a function of theimpedance between selected electrodes, the present invention uses ameasure of the lead impedance as an indicator of the electrode'seffectiveness in providing therapeutic stimulation. Thus, as an eventoccurs, such as a postural change, lead array movement, or scar tissuematuration, that allows more energy to couple from the emittingelectrode or electrodes to the neural tissue, the more likely thestimulus will be painful, i.e., the more likely over-stimulation willoccur. Conversely, as an event occurs that attenuates the coupledstimulation energy, the more likely the stimulus will not be sufficientto evoke the desired therapeutic effect (under stimulation).

As indicated above, the present invention uses the measured impedance asan indicator of stimulation energy delivered from the electrode orelectrodes to the target neural tissue. The measurement of impedancechanges between the stimulation electrodes or neighboring electrodesprovides a relative quantitative measure of the electrode array'stheoretical effectiveness in providing therapeutic stimulation. Therelative change in the measured impedance provides information on theability of electrical current to flow to the neural tissue. Knowledge ofthe measured impedance with respect to time allows for the system toeffectively auto correct the output amplitude, thereby minimizing theoccurrence of over-stimulation or under-stimulation.

One application for the present invention is for a Spinal CordStimulation (SCS) system that automatically corrects or adjusts thestimulation amplitude in order to maintain a comfortable and effectiveparesthesia. As postural change, lead array movement, scar tissuematuration, and the like occur, allowing more energy from the emittingelectrode or electrodes of the SCS system to be delivered to the neuraltissue, the more likely over-stimulation occurs. Conversely, as eventsoccur that attenuate the coupled energy, the more likely a desiredparesthesia does not occur (under stimulation). The relative change inmeasured impedance is used as a measure of the stimulation energydelivered to the neural tissue. Availability of the measured impedancechanges with respect to time allows the system to effectively autocorrect the stimulation output amplitude, thereby minimizing theoccurrence of over-stimulation (excessive threshold paresthesia) orunder-stimulation (sub-threshold paresthesia).

In accordance with one aspect of the invention, there is provided aneural stimulation system having means for measuring the impedancechanges between selected electrodes that occur over time in the epiduralspace where the electrodes are positioned to be in close proximity tothe dura mater of the spinal cord. The system also can provide means forcorrelating the change in impedance with downward or upward adjustmentof stimulation energy. Alternatively, the morphology of the impedanceresulting from an event, such as a postural movement may be correlatedwith the corrective adjustments to stimulation energy. Based on thispredetermined correlation, relative impedance changes of significance,i.e., changes indicating that over-stimulation or under-stimulation islikely to occur, are used as triggers to automatically make adjustments,or corrections, in the amplitude of the stimulus current to preventpainful over-stimulation or sub-threshold under-stimulation.

By way of example, as tissue healing (scar maturation) occurs, theimpedance may increase, requiring a higher output stimulus to achievethe same therapeutic result. The time frame is a slowly changingincrease in impedance with a corresponding increase in the amplitude (ormagnitude) of the stimulus output. Hence, the system of the presentinvention measures the impedance, e.g., usually a heavily weightedimpedance average with respect to time, in order to ascertain changes ofsignificance, e.g., changes of more than about 5-10% in the impedanceaverage compared with the impedance average obtained during a referencetime in the past. In response to such a slowly moving sensed changes inimpedance, the system automatically adjusts the output current orvoltage in order to maintain a desired therapeutic effect, e.g., tominimize sub-threshold stimulation or painful over stimulation.

By way of another example, a rapidly changing impedance variation wouldlikely indicate an acute movement of the electrode array surface contactfrom the proximity of the dura mater. Such a rapid change could be dueto a postural change causing the electrode array to move with respect toits location in the epidural space. In accordance with the presentinvention, when a significant change in average or other weightedmeasure of impedance, e.g., 10-15%, occurs over a relatively shortperiod of time, e.g., over 10-20 minutes, then such change triggers acorresponding change in the magnitude of the stimulus output in order tomaintain a desired therapeutic effect, e.g., to minimize sub-thresholdstimulation or painful over stimulation.

In another aspect of the invention an implantable neural stimulator isprovided, for autocorrection of stimulation. The stimulator comprises:means for measuring impedance indicative of the coupling efficiency ofan electrical stimulation current applied to neural tissue; and meansfor automatically adjusting the stimulation energy of subsequentstimulation in order to compensate for variations in the measuredcoupling efficiency.

In yet another aspect of the invention, a method is provided forautocorrection of neuro stimulation, the method comprising: measuringimpedance indicative of the coupling efficiency of an electricalstimulation current applied to neural tissue; correlating the decreaseor increase of the impedance with an increase or decrease in thestimulation energy delivered to the target neural tissue; andautomatically adjusting the subsequent stimulation energy delivered tothe neural tissue in order to compensate for variations in the measuredcoupling efficiency in order to maintain an optimal level of therapy. Tomeasure impedance the following steps may be taken: applying a stimulushaving a known amplitude to a first electrode in close proximity to theneural tissue; measuring a voltage developed on the first electrodewhile the current stimulus is applied thereto; and calculating theimpedance as the ratio of the measured voltage to the known current.

It is thus a feature of the present invention to provide a neuralstimulation system wherein the output stimulus amplitude isautomatically corrected or adjusted in order to compensate for couplingvariations in the electrode-to-neural-tissue interface that cause moreor less energy to reach the neural tissue from the electrode. In apreferred embodiment, such sensed coupling variations are determined bymeasuring or monitoring changes in impedance that occur between selectedelectrodes.

It is a further feature of the invention to provide a method of neuralstimulation that includes measuring impedance changes near the locationwhere a stimulus electrode array is located, wherein such impedancechanges are indicative of the coupling efficiency of the electricalstimulation current to the neural tissue, and automatically adjustingthe magnitude of subsequent stimulating current pulses in order tocompensate for variations in the measured coupling efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows a representative neural stimulation system of the type withwhich the present invention may be used;

FIG. 2 shows the stimulation system of FIG. 1 being used as a SpinalCord Stimulation (SCS) system, with the electrode array insertedalongside the spinal cord in the epidural space, in close proximity tothe dura mater;

FIG. 3A is a block diagram of a system that automatically adjusts theamplitude of the stimulus current applied to neural tissue in accordancewith the present invention;

FIG. 3B illustrates one method for sensing or measuring impedance inaccordance with the invention;

FIG. 4 is a block diagram of a representative Implantable PulseGenerator (IPG) that may be used to practice the present invention;

FIG. 5 is a timing waveform diagram that depicts one method formeasuring impedance; and

FIG. 6 is a high level flow chart that shows a method of practicing theinvention in accordance with one embodiment thereof.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

A representative neural stimulation system 10 is shown in FIG. 1. Such asystem typically comprises an Implantable Pulse Generator (IPG) 12, alead extension 14, an electrode lead 16, and an electrode array 18. Theelectrode array includes a plurality of electrode contacts 17 (alsoreferred to as “electrodes”). The electrodes 17 are arranged, forexample, in an in-line array 18 near the distal end of the lead 16.Other electrode array configurations may also be used. The IPG 12generates stimulation current pulses that are applied to selected onesof the electrodes 17 within the electrode array 18.

A proximal end of the lead extension 14 is removably connected to theIPG 12, and a distal end of the lead extension 14 is removably connectedto a proximal end of the electrode lead 16. The electrode array 18, isformed on a distal end of the electrode lead 16. The in-seriescombination of the lead extension 14 and electrode lead 16, carry thestimulation current from the IPG 12 to electrodes of the electrode array18. It should be noted that the lead extension 14 need not always beused with the neural stimulation system 10. The lead extension 14 isonly needed when the physical distance between the IPG 12 and theelectrode array 18 requires its use.

Turning next to FIG. 2, the neural stimulation system 10 is shown beingused as a Spinal Cord Stimulator (SCS) system. In such configuration,the lead 16, and more particularly the electrode array 18 is implantedin the epidural space 20 of a patient so as to be in close proximity tothe spinal cord 19. Due to the lack of space near the lead exit point 15where the electrode lead 16 exits the spinal column, the IPG 12 isgenerally implanted in the abdomen or above the buttocks. The leadextension 14 facilitates locating the IPG 12 away from the lead exitpoint 15.

A more complete description of an SCS system may be found in U.S. patentapplication Ser. No. 09/626,010, filed Jul. 26, 2000, now issued as U.S.Pat. No. 6,516,227, which patent is assigned to the same assignee as isthe present application, and is incorporated herein by reference in itsentirety.

Next, with respect to FIG. 3A, there is shown a functional block diagramof a system that automatically adjusts the amplitude of the stimuluscurrent applied to neural tissue in accordance with the presentinvention. As seen in FIG. 3A, an electrode 17 is placed in closeproximity to neural tissue 24 that is to be stimulated. The electrode 17is electrically connected to a current pulse generator 13 whichgenerates a stimulus pulse having a magnitude that is set by magnitudeadjust circuitry 115. The magnitude adjust circuitry 115 sets themagnitude of the stimulus pulse as specified by stimulation controlcircuitry 117. The stimulation control circuitry 117 usually comprisessome sort of processor, or state logic, that operates in accordance witha stored program or state diagram. It initially sets the magnitude ofthe stimulus pulse to a programmed or predetermined value.

As the stimulus pulse is applied to the neural tissue 24, an appropriateimpedance sensor S senses the coupling efficiency between the stimuluscurrent and the neural tissue. That is, the impedance sensor S providesa measure of how effective the applied stimulus is at stimulating theneural tissue 24. The sensor S is connected to the magnitude adjustcircuitry 115 to provide a feedback signal that indicates whether themagnitude of the stimulus needs to be adjusted up or down. For example,should the impedance measurement with sensor S indicate that very littleof the energy is being coupled to the neural tissue 24, then thefeedback signal provided through the sensor S automatically causes themagnitude adjust circuitry 115 to increase the magnitude of the stimuluspulse so that the total stimulation energy delivered to the neuraltissue 24 remains about the same. Conversely, should the impedancemeasurement with sensor S indicate that more energy is being coupled tothe neural tissue 24, then the feedback signal provided through thesensor S automatically causes the magnitude adjust circuitry 115 todecrease the magnitude of the stimulus pulse so that the total energydelivered to the neural tissue 24 remains about the same. Thus, it isseen that the magnitude adjust circuitry 115 automatically adjusts themagnitude, e.g., the amplitude of the stimulus pulse, so that thestimulation energy delivered to the neural tissue remains more or lessthe same.

It may be appreciated that while stimulation energy is generallydetermined by adjusting stimulus amplitude while holding pulsewidth andthe pulse frequency (pulses per second) constant, it is also possible tovary the stimulation energy by changing one or more of the stimulusparameters: pulse amplitude, pulsewidth, and frequency.

In accordance with the present invention, coupling efficiency isdetermined by measuring the impedance between selected electrodes in thevicinity of the electrode-tissue interface. Such impedance measurementcan provide a quantitative measure of the stimulating electrode'scoupling efficiency between the electrode and neural tissue. Forexample, as the impedance increases, that means the current flow to thetargeted tissue will likely decrease, thereby making the couplingefficiency lower. Conversely, as the impedance decreases, that means thecurrent flow to the targeted tissue will likely increase, thereby makingthe coupling efficiency higher. Thus, for purposes of the presentinvention, the sensor S measures impedance associated with the flow ofcurrent through the surrounding tissue, including the targeted tissue.

Next, with respect to FIG. 3B, there is shown one method for measuringimpedance. FIG. 3B shows a schematic representation of the electrodes 17a and 17 b located at a distal end of a lead. The body of the lead isnot shown in FIG. 3B, but it is to be understood that the electrodes 17a and 17 b are carried on the lead body, usually near the distal tip,and that the wires 19 a and 19 b, or equivalent conductors, that attachto the electrodes 17 a and 17 b, are embedded within the lead body, andexit the lead body near a proximal end, where connection can be madewith appropriate electrical circuitry. In a typical neural stimulationsystem, the lead will have at least one electrode contact, and willusually have a plurality of electrode contacts, e.g., four, eight,sixteen or more electrode contacts. The two electrode contacts 17 a and17 b shown in FIG. 3B are intended to be illustrative only, and notlimiting. The electrode contacts 17 a and 17 b are positioned in theepidural space near the dorsal column nerve fibers 24 within or near thespinal cord.

In operation, a stimulation pulse, from a pulse generator 30, is appliedto a selected pair of the electrode contacts, e.g., electrode contacts17 a and 17 b. As connected in FIG. 3B, the polarity of the pulsegenerator 30 causes a current, represented by arrow 28, to be emittedfrom electrode contact 17 a and to travel to the neural tissue 24. Thecurrent 28 flows through the nerve tissue 24 and surrounding tissue andreturns to the pulse generator 30 through electrode contact 17 b. Theenergy contained within the current 28 is coupled to the neural tissue24 as a function of the coupling efficiency between electrode Contacts17 a, 17 b and the neural tissue 24. This coupling efficiency may vary,for many reasons, such as postural changes, relative movement betweenthe electrodes 17 a and 17 b and tissue 24, or scar tissue maturation,to name just a few.

The electrodes 17 a and 17 b typically fit snugly within the epiduralspace next to the spinal column. Because the tissue is conductive, thereis an impedance, Z, associated therewith that reflects how easily thecurrent flows therethrough. Such impedance provides a good measure ofthe coupling efficiency between the electrodes and the neural tissue,and thus also provides a relative quantitative measure of the electrodearray's theoretical effectiveness in providing therapeutic stimulation.

The impedance may be sensed in various ways. One way is to measure theamount of current that flows through, e.g., conductor 19 a or conductor19 b, in response to a known potential or voltage that is placed acrossthe conductors 19 a and 19 b, and then calculating the impedance Z asthe ratio of known voltage to measured current. Another way, depicted inFIG. 3B, is to apply a known current, e.g., from a current source 30, toelectrode 17 a through conductor 19 a, with a return path throughelectrode 17 b and conductor 19 b, and then measuring the voltage V,using measurement or buffer amplifier 228, that is developed onconductor 19 a as a result of such current flow. The impedance Z maythen be calculated by controller 26 as the ratio of measured voltage Vto known current I. This impedance may then be stored in memory forlater reference, so that the next time an impedance measurement is made,a determination can be made as to whether the impedance has changed, andif so, how much, and what the rate of change is.

It is to be emphasized that the technique shown in FIG. 3B for sensingor measuring impedance is only representative of various ways that maybe used. Advantageously, the technique requires no separate sensor, perse, but uses the stimulation electrodes already carried on the leadbody. However, if needed, separate electrodes may be carried on the leadbody, or carried on another lead body, so that impedance measurementscan be made completely independent of the stimulation lead.

Turning next to FIG. 4, there is shown a functional block diagram of arepresentative Implantable Pulse Generator (IPG) 12 (or, with respect toFIG. 3, pulse generator 30) that may be used to practice the presentinvention. As seen in FIG. 4, the IPG 12 is connected to a multiplicityof electrode contacts E1, E2, . . . En, where n represents an integer ofat least 2. The dotted-dashed line 102 in FIG. 4 represents the boundarybetween the outside of the IPG case, which is exposed to body tissuesand fluids when the IPG is implanted, and the inside of the IPG case.The case forms an hermetically sealed compartment wherein the electronicand other components are protected from the body tissues and fluids.Feed-through terminals 104 a, 104 b, . . . 104 n are thus used toprovide an electrical path through the IPG case wall 102. Thefeed-through terminals 104 a, 104 b, . . . are electrically connected tothe electrodes E1, E2, . . . En through wires within the lead 16.

Thus, it is seen that each electrode contact E1, E2, . . . En isconnected through a respective feed-through terminal 104 a, 104 b, . . .104 n to a respective circuit node 106 a, 106 b, . . . 106 n within thehermetically sealed IPG case. This node, in turn is connected to a P-DACcircuit 108 a and an N-DAC circuit 110 a. Each of the other circuitnodes 106 b, . . . 106 n within the IPG similarly have a respectiveP-DAC circuit and N-DAC circuit connected thereto. The P-DAC circuitsare used as a programmable current source to generate a precise currentvalue that is emitted from the node to which the P-DAC circuit isconnected. Similarly, the N-DAC circuits are used as a programmablecurrent sink that receives a precise current through the node to whichthe N-DAC circuit is attached. Thus, in operation, one or more P-DACcircuits are programmed to emit a desired current value at the same timethat one or more N-DAC circuits are programmed to receive the samedesired current value.

A case electrode, CASE, may also be provided that effectively provides acommon or return electrode that may be used with some stimulation andsensing configurations, as required.

In operation, in order to generate a stimulus current pulse that isapplied between electrodes E1 and E2, for example, the P-DAC circuit 108a, as controlled by control logic 120 over data bus 122, causes astimulation current having a specified amplitude to be emitted from thenode 106 a, and hence to be emitted from the electrode contact E1. Atthe same time, the N-DAC circuit 110 b, similarly controlled by controllogic 120, causes a stimulation current of the same magnitude to bereceived through node 106 b, and hence through electrode contact E2.Although not shown in FIG. 4, coupling capactors connect the respectivenodes 106 and feed-through terminals 104. With the circuitry describedabove, a precisely controlled current is generated that flows fromelectrode contact E1 to electrode contact E2 through whatever body andnerve tissue resides between electrodes E1 and E2. The duration of thecurrent flow, i.e., the width of the current pulse that is generated, iscontrolled by timer logic circuitry 124. The operation of this outputcircuitry, including alternative embodiments of suitable outputcircuitry for performing the same function of generating currentstimulus pulses of a prescribed amplitude and width, is described morefully in the above-referenced U.S. patent application Ser. No.09/626,010, now issued as U.S. Pat. No. 6,516,227.

Voltage sense amplifiers 112 a, 112 b, . . . 112 n are also connected tothe respective nodes 106 a, 106 b, . . . 106 n. These sense amplifiersare used to measure the voltage appearing at selected nodes when a knowncurrent is flowing through the node. Such voltage measurements, incombination with the known current, allow the impedance to be readilycalculated by the monitoring circuitry 126 or processing circuitry 130.Typically, as described more fully below in conjunction with FIG. 5, theimpedance measurement is made on a sampled basis during a portion of thetime while the stimulus pulse is being applied to the tissue, orimmediately subsequent to stimulation. The impedance measurementprovides a measure of the coupling efficiency between the electrodesselected to perform the stimulation. The monitoring circuitry 126 alsomonitors other signals 128 from various locations or components withinthe IPG, e.g., battery voltage, charge current, etc.

The control logic 120, the timer logic 124, and the monitoring circuit126 are controlled or watched by a suitable micro-controller (μC)circuit 130. The μC circuit 130 is coupled to the control logic 120, thetimer logic 124, and the monitoring circuitry 126 over data buses 132,134 and 136, respectively.

Suitable memory circuitry 140 is likewise coupled to the μC 130, as isan oscillator and clock circuit 142. The μC 130, in combination with thememory circuit 140 and oscillator and clock circuit 142, thus comprise amicroprocessor system that carries out a program function in accordancewith a suitable program stored in the memory 140. Alternatively, forsome applications, the function provided by the microprocessor systemmay be carried out by a suitable state machine.

Power for the IPG is provided by a suitable power source 144, such as arechargeable battery or primary battery. A power circuit 146 can be usedto control the charging or replenishment of the power source, asdescribed more fully in the above-referenced patent application Ser. No.09/626,010, now issued as U.S. Pat. No. 6,516,227.

The power circuit 146, the μC 130 and the monitoring circuitry 126 arealso coupled to charging and telemetry circuitry 148. An antenna coil150 is likewise coupled to the telemetry circuitry 148. It is throughthe antenna coil 150 that charging, forward telemetry and back telemetrysignals may be received and sent to an external device, such as anexternal programmer or charging circuit, as described more fully in theabove-referenced U.S. Pat. No. 6,516,227. In practice, separate coilsmay be used for charging, forward telemetry and back telemetryfunctions, as described more fully in the above-referenced '227 patent,but for purposes of the present invention those distinctions are notimportant.

In FIG. 4, the antenna coil(s) 150 is shown as being outside thehermetically sealed case of the IPG. In such configuration, feed-throughterminals 103 are used to allow the coil(s) to be electrically connectedto the charging and telemetry circuitry 148, which are inside thehermetically sealed case. Alternatively, if the case is made from anon-ferromagnetic material, such as titanium, or ceramic, the coil(s)150 may be located inside of the hermetically sealed case.

It is to be emphasized that the schematic illustration shown in FIG. 4is intended to be functional, and not limiting. Those of skill in theart will be able to fashion appropriate circuitry, whether embodied indigital circuits, analog circuits, software and/or firmware, orcombinations thereof, in order to accomplish the desired functions.

FIG. 5 is a timing waveform diagram that depicts one way in which theimpedance measurement is made on a sampled basis. As seen in FIG. 5, attime t1, a stimulus current waveform “I” is generated. The waveform “I”is typically a biphasic pulse comprising a negative pulse followed by apositive pulse. The biphasic pulse has a width W. If the stimulus pulseis applied on a recurring basis, e.g., at a set frequency, then there isalso a period T between pulses, where the frequency is equal to 1/T.Usually, the biphasic pulse is symmetrical, and the positive andnegative portions of the pulse can have an equal width W/2.

As the biphasic stimulus pulse is applied to the tissue, a voltage V isdeveloped across the electrodes through which the current is flows. Arepresentation of such voltage V is shown in the middle waveform of FIG.5. At an appropriate sample time during the time when the stimuluscurrent pulse “I” is still being applied, e.g., at time t2 near the endof the positive pulse half of the biphasic pulse, the voltage V ismeasured. The impedance Z is then calculated as Z=V/I.

The impedance measurement will typically be processed in an appropriatemanner, e.g., averaged, so that a reliable determination of impedancecan be made. Other signal processing techniques such as statisticalsignal processing techniques may be used that weigh measurements nearthe average higher than measurements that are far away from the average,as are known in the art. Impedance measurements may also be determinedas a bipolar electrode configuration measurement or a monopolarmeasurement. Such measurements, and other aspects of measuringimpedance, are described in co-pending patent application, entitled“Methods for Determination of the Relative Position and Orientation ofNeurostimulation Leads”, Ser. No. 60/338,331, filed 4 Dec. 2001, whichapplication is assigned to the same assignee as is the presentapplication, and which application is incorporated herein by reference.

In accordance with the present invention, and in response to sensing asignificant change in the impedance measurement over time, correctioncircuitry programmed or wired into the μC 130 causes the amplitude ofthe stimulation current to also change by a compensating amount.

In one embodiment, the μC 130, or equivalent circuitry, is programmed toascertain whether the change in impedance is a slow change, which can becaused by scar maturation or fatty tissue build-up. Scar tissue canaffect the impedance in certain electrode configurations, e.g., a twolead system with an electrode on each, by changing the distance betweenelectrodes which are used to measure impedance and, in some cases, usedas stimulation leads. In general, scar tissue has approximately the sameimpedance as other living tissues such as fascia and muscle. Thus, it isnot the impedance of the scar tissue per se that is usually important,but the fact that development of such scar tissue can push and displaceelectrode positions relative to each other and relative to the positionof the target tissue, which thereby alters the coupling efficiencybetween stimulation energy emitted from the surface of an electrodecontact and stimulation energy actually delivered at the site of aneural tissue. Fatty tissue may change the long-term impedance also bydisplacing the electrodes and also sometimes by displacing surroundingfluids which are usually more conductive.

A rapid impedance change can be caused by a temporary postural change orby an acute lead movement. Slow changes in impedance are compensated forby making corresponding slow changes in the amplitude of the stimulationcurrent. Rapid changes in impedance, once confirmed through appropriatestatistical processing, are similarly compensated for by making rapidchanges in the stimulation current, but with automatic rechecking toascertain whether the change in impedance is temporary or permanent.

It is the energy content of the stimulus pulse that is adjusted inaccordance with the invention when a change in the coupling efficiencyhas been detected. The energy content of the stimulus pulse is readilyadjusted by adjusting the amplitude of the stimulus pulse. However, theenergy content can also be adjusted by changing the width, or duration,of the stimulus pulse waveform, as well as the frequency with which thestimulus pulse is applied. Thus, as used herein, the term “amplitude” or“magnitude”, although commonly used to signify a change in value of apulse, may instead be understood in the present context to broadly meanthe energy content of a stimulus pulse or a train of pulses. Thus, asused herein, a change of stimulus “amplitude” or “magnitude” can occurby changing pulse amplitude, pulsewidth, frequency of the pulses, or anycombination thereof.

FIG. 6 is a high level flow chart that shows a method of practicing theinvention in accordance with one embodiment thereof. A first stepinvolves Programming the operating parameters (block 202) into the IPGcircuitry. Such operating parameters include not only the operatingregime for the neural stimulation system, e.g., stimulation pulseamplitude, pulsewidth, frequency, and electrodes, but also theparameters used by the invention to determine when a sufficient changein the sensed impedance has occurred to trigger the auto correctionfeatures of the invention.

As a preliminary matter, because various electrode types may be used incombination with different electrode configurations, it is preferredthat each individual patient undergo a lab analysis which correlates thesensed impedance changes with a predicted magnitude adjustment to thestimulation emanating from the selected at least one electrode in thearray of electrode E1 . . . En. In a simple case, it is determinedwhether an increase in impedance (comparing at least two impedancesamples) corresponds to an adjustment requiring an increase or decreasein the stimulation energy. To obtain this correlation data, the patientcan be instructed to make various positional changes while a teststimulus current, similar to that shown in FIG. 5, is applied through astimulating electrode and responsive impedance measurements can berecorded as a function of the positional changes. In addition, it may benecessary to vary the stimulus during the assessment test to determinethe just noticeable stimulation (“perception threshold”) at a particularpatient position and the “maximum comfortable” level of stimulation inorder to interpolate and correlate the impedance change with thecorresponding adjustment to be made to the stimulation energy deliveredto the neural tissue. This correlation data can be stored into memory140 or other long-term memory in the IPG and can be recalled later toanalyze the indication of each sensed change in impedance.

A more sophisticated method for correlating an event change with achange in impedance uses impedance morphology or the plotted curve ofthe impedance as a function of time. An event that causes a short-termimpedance change may have a characteristic resulting impedancemorphology. For example, impedance changes caused by body movements mayhave different impedance morphologies. If an impedance morphology can belinked to an associated corrective adjustment, when that same morphologyis sensed by the IPG, the proper corrective adjustment may be appliedautomatically.

A specific method of implementing this link is to generate a correlationtable (“look-up” table) which may be developed for different bodymovements, for example. A short-term postural change (“an event”) mayoccur over an event time, T_(e). The types of postural events thatshould be included in the catalog of short term body movements are thosemovements normally made during the day, e.g., sitting to lying, standingto lying, lying to standing, sitting to standing, twisting the torso toone or the other side, standing to sitting (in a chair) and vice versa,standing to sitting (on a stool) and vice versa, running etc. Each ofthese events must be characterized in the laboratory for each individualpatient to generate a personalized look-up table that correlates thebody movements/events with a characteristic impedance waveformmorphology, over a time, T_(e). In addition, for each eventcharacterized, there is linked a corresponding predicted adjustments (upor down) to the stimulation energy, which may be adjusted, throughmagnitude adjust 115, by varying pulse amplitude, pulsewidth, and pulsefrequency.

To obtain usable impedance morphology associated with each bodymovement, it is necessary that a sufficient number of impedance valuesare sampled with the event time, T_(e). The IPG 12 can have automaticcapability to capture impedance (V/I) datapoints over a recording orsampling duration, T_(r), which should be greater than T_(e) to capturethe entire waveform. These data may be uplinked to an externalprogrammer (not shown) having processing capability or, alternatively, acomputer which can be used to analyze and set the corrective stimulusadjustments. After the entire look-up table is generated, includingcorrective stimulus adjustments linked one-to-one to each storedcharacteristic morphology, the data table can be downloaded to the IPGto be stored in memory. In accordance with the method of the presentinvention, this look-up table may then be recalled by software in theIPG to make nearly instantaneous corrective adjustments to stimulationenergy as a function of an identified short-term, impedance morphology.

Once all the operating parameters have been programmed, a determinationis made as to whether the auto correction feature of the invention hasbeen programmed ON (block 204). If not, then the stimulator operates inaccordance with its programmed operating regime without invoking theauto correction feature (block 206). Unless such programmed operatingregime is turned off (block 208), this process continues.

If auto correction is turned ON (block 204), then an impedancemeasurement is made (block 214), e.g., by sampling the voltage on theelectrode at a time when a known current is flowing to the electrode.Once an impedance measurement has been made, then a determination ismade as to whether the impedance has changed (block 216). If not, thenthe operation of the stimulator continues in accordance with itsprogrammed parameters (block 206), and the process continues.

If a determination is made that the impedance (size and/or morphology)has changed (block 216), then the operating parameters, e.g.,stimulation current amplitude, are adjusted to compensate for the changein energy coupling (block 218), and the stimulator is operated inaccordance with such newly adjusted parameters (block 206). Theselection of the stimulus amplitude (or energy) may be based onpredetermined calculations or based on a pre-programmed lookup table aspreviously described. In this manner, a negative feedback is providedthrough use of the impedance measurement to maintain the energy couplingand the energy delivered to the neural tissue at substantially aconstant or programmed level. The set level for maintaining the energydelivered to the neural tissue may be a pre-programmed single level orit may be a pre-programmed stimulation energy range that provides aresponse that is above the just noticeable stimulation (“perceptionthreshold”) and below the maximum comfortable stimulation. Stimulationenergies above the maximum comfortable stimulation level will causepain.

It is emphasized that while the present invention has been explainedspecifically in the context of the SCS application, the presentinvention can be practiced where the nerve stimulated is a peripheral,as well as a spinal cord nerve. All figures except FIG. 2 apply equallyto the case of stimulating peripheral nerve. FIG. 1 shows an in-lineelectrode array 18 whereas, in comparison, with a peripheral nerveapplication a different lead type and electrode configuration may beused. Peripheral nerve application also differs from SCS application inthat the results of stimulation provide different physical results. InSCS, sensory fibers are stimulated. In peripheral nerve application,sensory nerves are stimulated which innervate arms and legs. The methodand system of the present invention is not dependent, however, on aspecific type of lead or electrode configuration used. Rather, thepresent invention can be applied equally effectively for the purpose ofautocorrecting stimulation energy applied to peripheral nervestimulation following a detected change in impedance. Thus, it is withinthe contemplation of the present invention to include within its scopethe application where peripheral nerve is stimulated.

As described above, it is thus seen that the present invention providesa neural stimulation system wherein the output stimulus magnitude isautomatically corrected or adjusted in order to compensate for couplingefficiency variations in the electrode-to-neural-tissue interface thatcause more or less energy to reach the neural tissue from the electrode.Variations in the coupling efficiency are determined, in a preferredembodiment, by sensing changes in electrode impedance in the vicinity ofthe electrode-tissue interface.

As further described above, it is seen that the invention provides amethod of neural stimulation that includes measuring the impedance at ornear the electrode-tissue interface, which measurements are indicativeof the coupling efficiency of the electrical stimulation current to theneural tissue, and which measurement may be used in a feedback loop toautomatically adjust the magnitude of subsequent stimulus pulses inorder to compensate for variations in the measured coupling efficiency.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A method of providing therapy to a patient usinga neurostimulation system that stores electrical parameter measurementsin correspondence with different postural events, comprising: applying astimulation energy output from the neurostimulation system to thepatient; measuring an electrical parameter in the patient; comparing themeasured electrical parameter to the stored electrical parametermeasurements; determining if a postural event in the patient hasoccurred based on the comparison; and adjusting the applied stimulationenergy output if the postural event has been determined to haveoccurred.
 2. The method of claim 1, wherein the neurostimulation systemstores stimulation energy output adjustments correlated respectively tothe stored electrical parameter measurements are stored in theneurostimulation system, and the method further comprises comparing themeasured electrical parameter to the stored electrical parametermeasurements, wherein the applied stimulation energy output is adjustedbased on the comparison.
 3. The method of claim 2, wherein thecorrelated stimulation energy output adjustments and electricalparameter measurements are stored in a look-up table.
 4. The method ofclaim 1, wherein the measured electrical parameter is an impedancechange.
 5. The method of claim 4, wherein the impedance change is acharacteristic impedance waveform morphology.
 6. The method of claim 1,wherein the postural event is one of sitting to lying, standing tolying, sitting to standing, twisting the torso to one or the other side,standing to sitting, and running.
 7. The method of claim 1, whereinstimulation energy output adjustment comprises adjusting one or more ofa pulse amplitude, pulse width, and pulse frequency of the appliedstimulation energy output.
 8. The method of claim 1, further comprisingapplying an electrical signal in the patient, wherein the electricalparameter is measured in response to the application of the electricalsignal.
 9. A method of providing therapy to a patient using aneurostimulation system, comprising: applying a stimulation energyoutput from the neurostimulation system to the patient; measuring anelectrical parameter in the patient; determining if a postural event inthe patient has occurred based on the measured electrical parameter,wherein the postural event is one of sitting to lying, standing tolying, sitting to standing, twisting the torso to one or the other side,standing to sitting, and running; and adjusting the applied stimulationenergy output if the postural event has been determined to haveoccurred.
 10. The method of claim 9, wherein the measured electricalparameter is an impedance change.
 11. The method of claim 10, whereinthe impedance change is a characteristic impedance waveform morphology.12. The method of claim 9, wherein stimulation energy output adjustmentcomprises adjusting one or more of a pulse amplitude, pulse width, andpulse frequency of the applied stimulation energy output.
 13. The methodof claim 9, further comprising applying an electrical signal in thepatient, wherein the electrical parameter is measured in response to theapplication of the electrical signal.